Amebiasis denotes the disease caused
solely by Entamoeba histolytica, although there
are two other morphologically identical Entamoeba species
that can also infect humans—Entamoeba dispar and Entamoeba
moshkovskii. E dispar is about 10 times
more prevalent than E histolytica in most endemic
areas for amebiasis.1 Recent reports suggest that
infection of E moshkovskii, which was once considered
a free-living amoeba, is also common in some parts of the world
such as Bangladesh, India, and Australia. Most E moshkovskii infections
occur concomitantly with E dispar or E
histolytica infections. Neither E dispar nor E
moshkovskii infections are associated with disease.2 Free-living
amoebic infections are discussed in Chapter 348.
Amebiasis occurs worldwide but is much more common in developing
nations. This is a result of contaminated water or food, leading
to fecal-oral spread of the cyst. The World Health Organization
estimates that E histolytica is second only to
malaria as a protozoan cause of death. For example, the 1988 Mexican
national serosurvey demonstrated serological evidence of E
histolytica infection in 8.4% of the population.1 Nearly
half of the children surveyed in a refugee camp in Dhaka, Bangladesh,
had evidence of infection by age 5. Amebic dysentery is most common
in grade-school children.3 Amebic liver abscess
is 10 times more common in men than in women, being most common
in men between the ages of 20 and 50 years.1,4 This
male sex predominance is not observed in children. Steroid treatment
and pregnancy appear to increase susceptibility to life-threatening
infection. In the United States, most infections occur in immigrants
or in those who travel to developing countries. Amebic liver abscess
may present clinically with symptoms more than 6 months after travel
to an endemic area. Residents of institutions for the mentally retarded
and HIV-infected individuals are also at greater risk of E
E histolytica is a protozoan with an invasive, motile
trophozoite and infectious cyst stages that is responsible for person
to person transmission of infection. The trophozoite varies in diameter
from approximately 10 to 60 μm, has a clear ectoplasm,
and a single nucleus. The cyst averages 12 mm in diameter and has
1–4 nuclei (Figs. 341-1 and 341-2). Humans are the only reservoir for E.
Histolytica. Cysts that are passed in the feces of infected
individuals survive in a moist environment for months. Following
their ingestion in contaminated food or water the cysts travel to
the small intestine, the multinucleated metacystic amoeba is activated
and emerges through a hole in the cyst wall, and immediately after
excysting it undergoes division into eight uninucleate trophozoites.
These organisms do not colonize the small intestine but are usually
carried to the cecum where they become established.
Trophozoite of E. histolytica/E. dispar in
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